1. Field of the Invention
The present invention relates to an apparatus for and a method of controlling backlight for a liquid crystal display (LCD) device in which an image represented by video signals given thereto is displayed at timing synchronized with a synchronizing signal.
2. Description of Related Art
The liquid crystal display device is used for various portable instruments, e.g., a digital still camera and a movie video camera. Since such portable instruments are powered by a battery or the like, it is particularly required to save electric power consumed by the instruments.
The liquid crystal display device includes a liquid crystal display panel having a backlight device (lamp) such as a fluorescent lamp or light emitting diodes (LEDs) on a rear side thereof. The backlight device emits light onto a rear surface of the liquid crystal display panel to illuminate. In a liquid crystal display panel having a backlight device, most power is consumed by the backlight device.
It is therefore an object of the present invention to provide a backlight control apparatus for use with a liquid crystal display device to save power consumed by the display.
A backlight control apparatus for use with a liquid crystal display device according to the present invention comprises a liquid crystal display panel for displaying, at timing synchronized with a synchronizing signal, an image represented by a video signal supplied thereto; a backlight device for emitting light according to a voltage applied thereto to illuminate the liquid crystal panel from the rear, and voltage controller (control means) for controlling, during a synchronization period of the synchronizing signal, the voltage applied to the backlight device, the voltage being lower during the synchronization period than during a period other than the synchronization period.
In accordance with the present invention, there is also provided a method suitable for the backlight control apparatus above. The backlight control method for use with a liquid crystal display panel in which an image represented by a video signal supplied thereto is displayed at timing synchronized with a synchronizing signal, comprising the steps of: illuminating the liquid crystal display panel from the rear by a backlight device for emitting light according to a voltage applied thereto; and lowering the voltage during a synchronization period of the synchronizing signal, the voltage being lower during the synchronization period than during a period other than the synchronization period.
In accordance with the present invention, during the period of synchronization of the synchronizing signal (the synchronization period does not necessarily mean all the synchronization periods but include one per a plurality of synchronization periods), the voltage applied to the backlight device is lower (inclusive of zero volt) than that applied thereto during a period other than the synchronization period. Since the voltage applied is decreased during the synchronization period, the power consumed by the backlight device is lowered. This consequently saves the power consumed by the liquid crystal display device.
The synchronization period generally means so called a blanking period. The period other than the synchronization period generally means a period in which a video signal for display appears and may be called a xe2x80x9cvideo signal periodxe2x80x9d or a xe2x80x9cdisplay periodxe2x80x9d.
The synchronization period of the synchronizing signal in which the voltage applied to the backlight device is kept reduced may be a horizontal synchronization (blanking) period of a horizontal synchronization signal or a vertical synchronization (blanking) period of a vertical synchronization signal. Moreover, it is not necessarily required that the synchronization period of the lower voltage strictly matches a synchronization period. The synchronization period of the lower voltage may be slightly wider or narrower than the synchronization period. To lower the voltage applied to the backlight device, it is also possible, in addition to simply decrease the voltage, to set the value thereof to zero volt.
The voltage applied to the backlight device may be controlled such that the voltage is reduced once in a plurality of synchronization periods such that the voltage in the period is lower than that in the periods other than the synchronization period.
When the voltage applied to the backlight device is decreased for every synchronization period, an image displayed on the liquid crystal display panel may flicker in some cases. However, the flicker can be suppressed by appropriately controlling the voltage applied to the backlight device, i.e., the lowering degree of the voltage.
It is also possible to dispose a voltage setting device to set the voltage lowering ratio or lowered voltage. The voltage applied to the backlight device in the synchronization period is determined according to a value (ratio or voltage) set by the voltage setting device.
The user may set a voltage lowering ratio of a voltage applied to the liquid crystal panel or the voltage as desired.
The backlight control apparatus may further comprises a power source low voltage detector to detect an event that the power source voltage becomes equal to or less than a predetermined threshold value and a voltage control unit (means) to cause the voltage controller to control the voltage applied to the backlight device during the synchronization period in response to the detection of the low voltage event by the detector.
When the power source voltage is decreased, that is, when the voltage control for power saving is actually required, the voltage applied to the backlight device is controlled. This advantageously saves the power consumption.
When the liquid crystal display panel operates according to power from an alternating current (ac) power source, it is not required to consider the power consumption in most cases. Therefore, the voltage applied to the backlight device need not be lowered even in the synchronization period above.
That is, the backlight control apparatus is further provided with an AC adapter connection (or mount) detector for detecting that the AC adapter which convert an ac voltage (power) to a direct current (dc) voltage (power) of a predetermined value has been connected (mounted). The voltage controller caused to be inactive or disable when the AC adapter connection is detected by the detector so that the control of voltage applied to the backlight device during the synchronization period ceases or is stopped.
Assume that the backlight control apparatus of the present invention is applied to a camera, especially an electronic still camera. To adjust an angle of view, it is necessary for the user to clearly identify or recognize an image displayed on the liquid crystal panel, and hence the voltage lowering control is temporarily stopped to prevent the flickering of the displayed image. For example, when at least a focusing operation (control) or an automatic exposure adjusting operation (control) is carried out in response to a first-step depression of a shutter release button, the voltage lowering operation (control) in the synchronization period is inhibited until an event of the first-step depression is detected. In response to the detection of the first-step depression of the shutter release button, focusing control, exposure control or the like is executed. In this situation, the user need not to clearly identify the image displayed on the display panel, and hence the voltage applied to the backlight device is lowered during the synchronization period, that is the voltage controller becomes active in response to the detection of the first-step depression of the shutter release button.